The prior art is replete with RF power amplifiers suitable for use with numerous practical applications. For example, mobile telephones and other wireless communication devices are common applications for RF power amplifiers. In mobile telephone applications, the RF output power level for the transmit signal may vary over time due to operating conditions and/or output power modulation schemes. An RF power amplifier becomes saturated when the change in output power decreases to zero with an increase in a control variable (e.g., an output power control voltage). Operation in a saturated condition may result in distortion and can compromise the operation of a closed loop control scheme for the RF power amplifier.
The term “soft saturation” refers to operation of an RF power amplifier in a region that precedes the actual saturation point. A number of detection techniques have been developed to detect the onset of soft saturation before serious distortion or control issues arise. For example, prior art techniques rely on the detection of a maximum triggering level of a control voltage signal in the RF power amplifier, below which the amplifier operation is unaffected by the effects of saturation. In practice, however, the triggering level for a given RF power amplifier can vary from unit to unit and even within a given unit over different operating conditions. Consequently, a fixed triggering level may not correspond to optimal soft saturation detection in all cases and these soft saturation detection techniques may rely on ambiguous detection thresholds. Such ambiguity may cause the detection scheme to overshoot or undershoot the actual onset of soft saturation in the RF power amplifier. Although undershooting the onset of soft saturation will not adversely affect the operation of the RF power amplifier, undershooting results in inefficient use of available output power. Undershooting in this manner will result from very conservative threshold levels, which require excessive headroom with lower battery efficiency for mobile applications. Otherwise, substantial amounts of calibration are required (e.g., phasing) during manufacture of the device. In contrast, overshooting the onset of soft saturation may result in actual hard saturation of the RF power amplifier and the associated distortion and control issues mentioned above.
Accordingly, it is desirable to have a soft saturation detection technique, suitable for use with RF power amplifiers, that unambiguously measures the amount of saturation occurring in the amplifier using signals available in the amplifier circuit. In addition, it is desirable to have a soft saturation detection circuit that provides an accurate and device independent measure of approaching saturation in an RF power amplifier. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.